Answer to Question #860 Submitted to "Ask the Experts"

Q

Does anyone else out there have false alpha continuous air monitor (CAM) alarms that seem to be correlated to temperature inversions and possibly accompanying high humidity?

A

This is a very common occurrence, especially when using continuous air monitors (CAMs) that do not have active radon compensation capabilities. Alpha CAMs that are "gross" counters (no radon compensation) are susceptible to fluctuations in the level of radon around the CAM. Atmospheric conditions, such as temperature inversions and/or high humidity levels tend to hold the radon down, creating higher-than-normal levels of radon. If the CAM doesn't have the ability to compensate for these conditions by either not having any radon compensation capability or, if it does have compensation capability but it is not flexible enough to be responsive to these conditions, the increased radon level will cause "false" alarms. I think it is important to emphasize that the CAM, whether it is a multi-channel analyzer-based system (compensating) or a gross counter, does not see counts in the channel, or region of interest, as false counts. If the CAM you are using uses a region-of-interest-based compensation technique, the increased level of radon will cause the net count rate (determined from the compensation algorithm) to become unstable. A change in the ratio between the 7.65 MeV and 6 MeV peaks may not be compensated correctly since most of these monitors are set up for "normal" conditions that may only exist at certain times of the day or year. Some CAMs use peak-shape analysis that may provide better compensation since they do not use ratios or compensation factors. We are in the process of evaluating this type of monitor. In summary, the problem you mention is common but can be minimized through calibration or field set up, or CAM design. When counts are observed in the channels or region of interest, the CAM sees this as activity of concern. Lastly, if the CAM you use has data storage capability, I would suggest that data from each radon daughter be tracked for a period of time (a few weeks maybe) so that the compensation factor can be properly determined. Peter Chiaro, Jr. Measurement Science Section Instrumentation and Controls Division Oak Ridge National Laboratory